In modern integrated circuit (IC) fabrication, layers of material are applied to embedded structures previously formed on semiconductor wafers. Chemical Mechanical Planarization (CMP) is an abrasive process used to remove these layers (or portions thereof) and polish the resulting surface to achieve a desired structure. CMP may be performed on both oxides and metals and generally involves the use of chemical slurries applied in conjunction with a polishing pad in motion relative to the wafer (e.g., the pad rotates relative to the wafer with the slurry dispersed therebetween). The resulting smooth flat surface is necessary to maintain photolithographic depth of focus for subsequent wafer processing steps and to ensure that the metal interconnects are not deformed over contour steps. Damascene processing requires metal, such as tungsten or copper, to be removed from the top surface of a dielectric to define interconnect structures, using CMP.
FIG. 1 shows a cross-section of a polishing pad 100 made by SemiQuest, Inc. This polishing pad is described in U.S. patent application Ser. No. 11/576,944, filed 9 Apr. 2007, assigned to the assignee of the present invention and incorporated herein by reference. Pad 100 consists of polishing elements 102, which rest on a compressible under-foam 104 and are supported in vertical orientation by a guide plate 106. An optional slurry distribution layer 108 may be disposed above the guide plate. Polishing action is provided by the polishing elements, which are made of solid polymer material, while slurry distribution is effected by the open spaces between the polishing elements.
As discussed in U.S. patent application Ser. No. 11/576,944, the ability to monitor process conditions while a wafer is being polished is important as it can provide information on the wafer surface, which, in turn, may be utilized to change the process conditions or stop processing all together. As is known in the art, some CMP systems use optical means to monitor process conditions. In particular, a light beam is directed toward the wafer through an open aperture in the polishing pad and reflected off of the wafer surface being polished. Changes in the reflected beam can be analyzed to determine the condition of the polishing process. Examples of systems, pads and methods for such process monitoring are described in U.S. Pat. Nos. 7,264,536, 7,374,477, 7,118,450, 7,029,747, 6,884,156, 6,524,164, 6,280,290, 5,893,796, 5,609,517, and 5,433,651, each incorporated herein by reference.